File Source: Sunspot data from the World Data Center SILSO, Royal Observatory of Belgium, Brussels

"Daily total sunspot number derived by the formula: R= Ns + 10 * Ng, with Ns the number of spots and Ng the number of groups counted over the entire solar disk.

No daily data are provided before 1818 because daily observations become too sparse in earlier years. Therefore, R. Wolf only compiled monthly means and yearly means for all years before 1818." (http://www.sidc.be/silso/infosndtot)